Darwin: fix SIGTRAP when debugging

Debugging a program under Darwin does not work:

(gdb) start
Temporary breakpoint 1 at 0x100000fb4: file /tmp/helloworld.c, line 1.
Starting program: /private/tmp/helloworld
[New Thread 0x2903 of process 60326]
During startup program terminated with signal SIGTRAP, Trace/breakpoint
trap.

Field signaled from darwin_thread_info is not initialized thus signal
sent to the debuggee is considered as not sent by GDB whereas it should.

This patch fixes this problem and also updates (change type and/or
initialize) other fields in the same structure at the same time.

gdb/ChangeLog:

        * darwin-nat.h (struct darwin_thread_info) <gdb_port,
	inf_port, msg_state>: Initialize.
        (struct darwin_thread_info) <signaled, single_step>: Change
        type and initialize.
        (struct darwin_thread_info) <event>: Initialize.

Change-Id: I0fe2a6985df9d0dfcc8a2a258a3ef70cfa19b403